Storage head for a blow molding machine

ABSTRACT

The invention relates to a storage head for a blow moulding machine for discontinuous manufacture of multi-layer co-extruded and blow-moulded hollow bodies made of thermoplastic. To improve the product quality and to increase the throughput, each distributor element is designed as a spiral channel distributor (20) which distributes the individual molten plastic streams uniformly at the periphery. The ejector (14) consists of at least two concentric tubular pieces (42, 44, 46) and the peripherally distributed plastic streams issue from the front face of the ejector (14) and unite to form a single multi-layer flexible plastic tube in the storage space (16) in the storage head housing (10) below the ejector.

BACKGROUND OF THE INVENTION

The invention relates to a storage head for a blow molding machine forthe discontinuous production of multiple-layer high-volume plastichollow bodies, comprising: at least two extruders connected to thestorage-head housing to feed at least two liquid plastic melts into thestorage head; a centrally disposed shaft; and at least two mutuallyindependent distributor elements, concentrically surrounding the shaftto distribute the plastic melts circumferentially and to conduct theminto an annular storage space below the ejector piston, which is mountedin the storage-head housing by means of a setbolt so as to be movableaxially, and by means of which the multiple-layer plastic melt which isdistributed circumferentially and which is stored in the storage spaceis ejected or co-extruded through an annular nozzle gap which isconnected below to the storage space.

A comparable storage head of this type is known, e.g , from the DE-OS 3902 270. This storage head is used for the discontinuous production ofmultiple-layer, co-extruded, hose-like parisons of thermoplastic to formhigh-volume, multiple-layer hollow bodies in a divided blow mold. Withthis storage head, at least two different annular material melts arebrought together centrally within the ring piston, one after the other,in the extrusion direction, to form a multiple-layer material melt.After this, the multiple-layer material melt expands in funnel-likefashion and flows into an annular storage space. Subsequently, it isejected from the storage head housing, by a ring piston that is movablein the axial direction, through an annular nozzle gap. The streams ofplastic melt are conducted individually for each layer - up to fivedifferent layers may be present. Each one is distributed over thecircumference by a separate ring channel. From there, they flow througha annular gap into the storage space beneath the ejector piston. Withthis circumferential distribution through a ring channel, adisadvantageous feature is the fact that on the side which is oppositeto the infeed point, where the two semicircular partial streamscoalesce, a welding seam always results, which later appears in theblown hollow body as a longitudinal seam and thus as a weak point. Inthe case of co-extruded, multiple-layer hollow bodies, the inner weldingseams are indeed always covered up by the outer layer; nevertheless,they remain recognizable in the finished product by the formation ofundesirable striations.

For example, ring channels or heart-shaped distributors are known asdistributor elements to distribute the solid melt strand of plastic,which is conducted from the extruder into the storage housing, onto acircular circumference. However, various plastic molecules or particleshere must traverse flow paths of different length. The coalescencepoints of the individual plastic streams may be recognizable in theblow-molded article as corresponding coalescence-based longitudinalseams. This reduces product quality.

SUMMARY OF THE INVENTION

It is an object of the invention to specify a novel storage head for ablow molding machine for the discontinuous production of plastic hollowbodies, especially high-volume ones, which has rheologically the sameflow conditions for all the plastic particles when the liquid meltedplastic stream is conducted to a distributor element, and in thedistributor elements themselves, and which makes possible improvedoverlap and circumferential distribution of the melt streams in thedistributor element while simultaneously increasing the throughput powerand production speed of the blow molding machine.

According to the invention, this object is achieved as follows: Theejector piston consists of at least two tubular pieces which surroundone another concentrically, and a separate distributor element isassociated with each tubular piece, each distributor element beingdesigned as a cylinder-shaped spiral-channel distributor.

With the inventive design, the plastic melts are fed in from theextruders by means of a spider shaft designed as a multiple-channelsystem. Here, the individual plastic melts are each divided from anaxial central stream (main stream) to several radial borings (partialstreams) which lead in star-shaped fashion to the outside. The centralseparate infeed and the melt distributors (spiral channels) which followso as to be connected symmetrically with respect to the flow directionguarantee a completely symmetric circumferential distribution of themelted plastic streams as far as the storage space of the storage head,where the individual plastic layers, distributed along thecircumference, combine to form a single, multiple-layer plastic hose.Here, each plastic particle at each location in the circumference of theco-extruded article has one and the same rheological flow history,independent of the throughput power of the extruder or of the storagehead. The consequence of this fact is that the all-around distributionis completely symmetric.

In a configuration of the invention, the arrangement is such that theaxial length of each distributor element or of the individual spiralchannels in the longitudinal direction is about the same as itsrespective circular diameter. Here, at least two or more spiral channelsare provided, uniformly distributed along the circumference of thedistributor element. The separate streams of plastic melt at first areeach conducted centrally through an axial boring, as the main streamwithin the shaft. The individual spiral channels are each then connectedas partial streams to the central boring, through radially extendingborings which are arranged in star-shaped fashion. The spiral channelsare thus supplied with plastic melt. The spiral-channel distributor canbe compared to a multiple-start thread or spiral channel with aparticular pitch, affixed on an outside wall (e.g. of the shaft, of theejector piston, or of an additional sheath), or on an inside wall (e.g.of the ejector piston, of an additional sheath, or of a cylindricallyshaped, tubular piece, or of the inside wall of the storage head). Thespiral channels are fed individually, and their depth decreases steadilyin the flow direction. In this way, the plastic streams existing in thespirals are gradually brought over into axial streams. The axial streamsform in the enlarging annular gap between the spiral-channel distributorand the sheath which covers it. The axially outflowing streams of meltfrom the individual spiral channels always overlap the axialcircumferential streams which flow underneath, as well as the residualstreams of melt which flow in the spiral channels. They do this as asickle-shaped circumferential surface. The individual spiral channelsare fed, as already mentioned, through a central boring in the shaft.Depending on the number of spiral channels, this is divided instar-shaped fashion outwardly among the individual spirals.

An essential feature of the invention is that the distributor elementsare designed as cylindrically shaped spiral-channel distributors(annular gaps). The combination of the spiral-channel distributors withthe respectively central infeed of the individual streams of plasticmelt offers quite decisive advantages:

The central infeed achieves a rheologically uniform flow history for allthe plastic particles. The spiral-channel distributor achievesfar-reaching overlap of the streams of melt with respectively only onemelt distributor.

The inventive storage head with a spiral-channel distributor increasesthe throughput power as compared to conventional storage heads with,e.g., a ring-channel distributors or heart-shaped distributors. Forexample, in the case of a 20 1 head storage, the throughput power isincreased from 650 kg/h to about 800 kg/h. Furthermore, it improves theall-around distribution of the plastic material.

There are natural limits to the increase of throughput power in the caseof known melt distributors, in which the ejector piston of the storagehead distributes the plastic material through heart-shaped elements.When these limits are exceeded, product quality is reduced, for example,by partial thin points or longitudinal seams in the article.

In the case of the inventive co-extrusion storage head, the ejectorpiston consists of several cylindrical sheaths or tubular pieces. Here,each tubular piece receives a spiral-channel distributor on its outsidesurface, and the inside surface of the tubular piece servessimultaneously as a fixed covering for the next interior spiral-channeldistributor. The outermost spiral-channel distributor is suitablycovered with a thin sheet metal sheath, fastened at the ejector piston,to prevent shear forces from acting on the plastic melt, as a result ofadhesion to the relatively vulnerable inside walls of the housing.

In an embodiment as a triple-layer co-extrusion storage head, threeseparate extruders feed in their plastic melt at a different heightthrough the respectively centrally situated infeed borings in the shaftfor the individual spiral-channel distributors. Different plastics canhere be combined even without a bonding agent.

Thus, the inside layer of a triple-layer parison can consist of ahigh-grade food-compatible new plastic material such as, e.g., HDPE--or,for filling with hazardous liquids, it can have diffusion blockingadditives, e.g. Celar platelets)--, while the thicker middle layer, as asupport layer, is formed of reprocessed recycling regranulate, and thethird outermost layer, as the dye-bearing layer, again consists of newmaterial charged with pigments.

When only two extruders are used for a triple-layer parison, the middlelayer suitably again consists of filler regranulate, while the inner andouter layers are supplied with plastic melt from only one extruder andconsist of new material.

The inventive spiral-channel distributor achieves the followingadvantages:

extensive area overlap of the individual melt streams with only one meltdistributor element,

overlap length possible beyond half the circumference (180°) up to about240° (important for welding the individual streams); in the design with,e.g., six spirals and 240° overlap length, this leads to quadrupleoverlap in a single layer

the same rheological flow properties hold for all the individualstreams,

since each spiral channel is fed individually (partial stream), a fasterand better circumferential distribution and thus a considerable increaseof throughput power can be achieved by means of a storage head that isequipped with spiral channels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an inventive storage head to produce a two-layer,co-extruded hollow body,

FIG. 2 shows a second inventive storage head to produce a triple-layer,co-extruded hollow body,

FIG. 3 shows a third inventive storage head and

FIG. 4 is a cross-sectional view taken along lines A-B-C-D of FIG. 3,

FIG. 5 is a cross-sectional view taken along lines E-F-G-H of FIG. 3,and

FIG. 6 is a cross-sectional view taken along lines I-J-K-L of FIGS. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, the housing of a storage head (accumulator head) for a blowmolding machine is designated by the reference number 10. Acylindrically shaped shaft 12 is disposed centrally in the storage headhousing 10. An ejector piston 14 is movably mounted on the shaft 12 bymeans of an hydraulically activated setbolt 18. On the outside, theejector piston 14 abuts flush-tight to the inside wall of the storagehead housing 10. Below the ejector piston 14 (ring piston), between theshaft 12 and the housing 10, a storage space 16 is formed for theintermediate storage of the plastic melt that is distributedcircumferentially. The multiple-layer plastic melt, which collects inthe storage space 16, slowly but steadily raises the ejector piston 14into its upper ejection position. When the storage space 16 is filled,the plastic melt that is in intermediate storage is ejected orco-extruded as a hose-shaped parison into an opened blow mold that isdisposed underneath, by the downwardly moving ejector piston 14, througha annular-gap nozzle 28 that is formed between the central nozzle core24 and the outer nozzle ring part 26. In the representation of FIG. 1,the ejector piston 14 is just situated at its lowermost position(ejection position).

According to the invention, the ejector piston 14 consists of threetubular pieces 42, 44 and 46, which surround one another concentrically,and which are all rigidly connected together. At the surface of theinner tubular piece 42 and the middle tubular piece 46, distributorelements are formed, with at least four circumferentially distributedspiral channels. The outermost tubular piece 44 here serves only as acover sheath for the outermost spiral-channel distributor.

Each of two liquid plastic melts is fed through the radial feedline(boring) 32, 32' from the two extruders, so as to pass centrally intothe shaft 12. There, the melt streams are deflected into an axial boring22, 22'. The length of the respective central borings 22, 22' in theshaft 12 are at least as long, but preferably about three times as long,as the diameter of the boring 22, 22' itself.

From the central borings 22, 22', the main melt streams are divided intoindividual partial streams, through a plurality of radial borings 30,30' which run in star-like fashion. These partial streams are conductedto an individual spiral groove of the respective spiral-channeldistributor. The transition of the respective partial streams of plasticmelt from the fixed shaft 12, through the radial borings 30, 30' to themovable ejector piston 14 is, in each case, made through a longitudinalgroove 34, 34' so as to bridge over the stroke motion.

According to a feature of the invention, at least one of the setbolts18--which connect to the ejector piston 14 and which are provided forthe axial motion of the ejector piston 14--are designed hollow and areprovided with an axial boring 38. This setbolt is used as an infeed lineto feed the plastic melt into a distributor element.

In the present case, only three out of the six setbolts is designedhollow. One partial stream from a radial boring 30 is conducted througha longitudinal groove 34 in the hollow setbolt 18 into a feed line 52 inthe ejector piston 14.

Each feed line 52 is forked once again in the circumferential direction(divided into two partial streams). It supplies the two adjoining spiralchannels with plastic melt, so that here six individual spiral channelsof the outer spiral-channel distributor 20' can be fed through threehollow setbolts 18.

The inner spiral-channel distributor 20 is supplied with plastic meltthrough longitudinal channels 34' which are disposed directly in theouter wall of the shaft. The individual, circumferentially distributedplastic streams exit frontally from the ejector piston or from theindividual distributor elements, as thin hose-like layers. They combinein the storage space 16 to form a multiple-layer, tubular plasticstrand, which is ejected or co-extruded discontinuously from the annulargap nozzle 28.

FIG. 2 shows as triple-layer co-extrusion head. The hollow setbolts 18and 18' (through the longitudinal grooves 34) are respectively suppliedwith plastic from an extruder through the radial boring 32 and thethen-following axial boring 22 in the shaft 12, and through the furtherfollowing radial boring 30 (partial streams). However, following this,the spiral-channel distributor 20' on the middle tubular piece 46 issupplied with plastic melt through three hollow setbolts 18, while thespiral-channel distributor 20" on the outer tubular piece 44 is suppliedthrough three other setbolts 18'. Through this inventive measure, too, aplastic material from an appropriately large extruder can be distributeduniformly on the circumference, simultaneously in two layers, muchfaster and better and with much higher throughput power.

The partial section on the left half of the drawing shows thespiral-channel distributor 20". To avoid shearing action, thisspiral-channel distributor is suitably covered by a thin metal sheaththat is not shown in the drawing. It is important for the structuraldesign of the spiral-channel distributors that the individual spiralsare disposed at a distance from one another. Here, the average distancehas about the same axial width as the width of a spiral groove. It isalso important that the width of the spiral grooves decreases slowly andsteadily with increasing length, and that the cylindrical area lying inbetween correspondingly increases and becomes wider. Here, the spiralgrooves are bounded laterally by defined edges. The lower edge or thedownwardly adjoining cylindrical surface is, in each case, set back alittle relative to the upper edge.

The spiral channels here have a semicircular cross-section, at least attheir beginning. As the length of the spirals increases, thissemicircular cross-section becomes flatter and flatter, and the depth ofthe spirals decreases more and more.

The representation of FIG. 3 shows the central feeds and thedistribution of the partial streams from three different plastic mainstreams to three spiral-channel distributors (triple-layerco-extrusion). Only in this way are the same rheological flow conditionscreated for all the plastic particles in the respective plastic streams.So that the structural height of such a storage head need not be madeneedlessly large, it may be suitable, at least in the case of onecentral boring 22, to dispose the feed of the plastic melt from theextruder via the radial borings 32 (main stream) into the end of boring22 that faces the ejector piston 16 and to connect in the same directionas the lower end of the star-shaped radial borings 30, which lead to thespiral channels, at the other end of the boring 22 which faces in adirection away from the the lower end of the piston. In this way, theplastic melt in this central boring 22 is conducted in a flow directionopposite to the ejection direction.

This is done here in connection with the central boring 22', which isdisposed in the middle. The radial borings 30', which follow saidcentral boring, merge on top and allow the partial streams to flowthrough the hollow setbolts 18' to a spiral-channel distributor 20",which is disposed on the outside sheath 44 (for the sake of simplicity,only shown in phantom in the drawing). The partial streams here flowinto the six individual spiral channels of the spiral-channeldistributor 20". (The nozzle core 24, the nozzle ring part 26 with theintermediate annular gap nozzle 28 likewise are not shown here.) Thepartial streams from the infeed borings 38' of the three hollow setbolts18' are again divided or forked the infeed lines 52' and thus can alwaysfeed two adjoining spirals.

As another inventive feature, FIG. 3 shows that the connection orjuncture between the hollow setbolts 18 as plastic infeed line and ashead area of the ejector piston is designed as a spherically shapedconnecting element with a spherically shaped head 48 and a sphericallyshaped shell 50.

The lower end of the setbolt 18 is here suitably designed as a sphericalhead 48. On the other hand, the corresponding spherical shell 50 isformed above, in the head area of the ejector piston. Naturally, thisjunction in principle would also be possible in the inverse design. Thisserves very simply to compensate tolerances and thermal stresses betweenthe long movable setbolts and the adjoining ejector piston as well asthe outer fixed components of the housing. Since the setbolts 18essentially sustain only pressure stresses, the spherical head 48 of thesetbolts can be fastened on the ejector piston simply by means ofannular pieces formed like hemispherical shells. With a multiple-layerco-extrusion head, the directions and courses of the spirals of thespiral channels can be designed differently for at least two adjoiningdistributor elements with spiral channels. In other words, they can bedesigned oppositely right-handed and left-handed.

In this way, one can achieve a crossing overlap of individualcircumferential layers with a resulting increase of strength, e.g.,against the interior pressure stresses from the blown hollow body.

FIG. 4 shows a cross-section through the upper part of the storage headaccording to the exploded plane A-B-C-D in FIG. 3. The plastic meltconducted from the extruder passes through the radial infeed boring 32into the axial boring 22 which is disposed centrally in the shaft 12. Toequalize the pressure due to the deflection from the boring 32, theaxial boring 22 should have about three times the length of itsdiameter. From the axial boring 22, the partial streams pass through thestar-shaped radial boring 30 into the longitudinal grooves 34, which arelaterally disposed in the six setbolts 18 (for stroke equalization).From there, they pass through the longitudinal borings 38 in thesetbolts 18 to the individual spiral channels. Here the other setbolts18', which are disposed symmetrically in between, are still designedsolid.

The cross-section in FIG. 5 runs in the exploded plane E-F-G-H in FIG.3. Here, the plastic stream passes through the radial infeed boring 32'from the extruder and into the axial boring 22'. It flows from thebottom to the top through the radial borings 30', which merge on topwith the axial boring 22'. It continues via the longitudinal grooves34', into the hollow setbolts 18'. The transition from the setboltboring 38' to the feed line 52' in the ejector piston 14, as alreadyexplained, takes place through a spherical-head juncture 48', 50'. Thethree feed lines 52' are forked once again and go over into six spiralchannels. The feed borings 38 in the setbolts 18 can be seen in the lefthalf of FIG. 5, while the longitudinal grooves 34 are cut in the righthalf of the drawing.

FIG. 6 shows the cut plane I-J-K-L from FIG. 3. The neighboring setbolts18, 18' together with their feed borings 38, 38' can be seen in the lefthalf of the drawing. The axial boring 22" is fed from a third extrudervia the radial boring 32'. It distributes six partial streams over thesmaller radial borings 30", via the longitudinal grooves 34" in theshaft 12, through the individual spiral grooves of the innerspiral-channel distributor 20.

The structural features shown in the various embodiments can bearbitrarily interchanged. With the inventive embodiments of a storagehead housing with spiral-channel distributors, plastic hollow bodies canbe produced with improved product quality while increasing theproduction speed.

LIST OF REFERENCE NUMBERS

storage-head housing

shaft

ejector piston

storage space

setbolt

spiral-channel distributor

central feed (axial boring)

nozzle core

nozzle ring part

annular gap (nozzle)

radial borings

infeed extruder

longitudinal groove

intermediate sheath

borings (18)

inner tubular piece

outer tubular piece

middle tubular piece

spherical head (18)

spherical shell (14)

feed line (to 20)

We claim:
 1. A storage head for a blow molding machine for thediscontinuous production of multiple-layer high-volume plastic hollowbodies, comprising: at least two extruders connected to a storage-headhousing (10) to feed at least two liquid plastic melts into an annularstorage space of a storage head; a centrally disposed shaft (12); and atleast two mutually independent distributor elements, concentricallysurrounding the shaft (12) to distribute the plastic meltscircumferentially of said shaft and to conduct them into said annularstorage space (16) below an ejector piston (14), which is mounted in thestorage-head housing (10) by means of one or more setbolts (18) so as tobe movable axially, and by means of which a multiple-layer plastic meltwhich is distributed circumferentially of said shaft and which is storedin the storage space (16) is co-extruded through an annular nozzle gap(28) which is disposed below the storage space (16), wherein the ejectorpiston (14) consists of at least two tubular pieces (42, 44, 46) whichsurround one another concentrically, and a separate one of saiddistributor elements is located on each said tubular piece (42, 44, 46),each said distributor element being defined by a cylinder shapedspiral-channel distributor (20).
 2. The storage head of claim 1, whereinthe axial length of each spiral channel distributor is about the same inthe longitudinal direction as its respective circular diameter, andwherein at least two spiral channel distributors are provided so as tobe uniformly located along the circumference of the distributor element.3. The storage head of claim 1, wherein the individual spiral channeldistributors are disposed at a distance from one another which has aboutthe same axial width as the width of one spiral channel distributor, andwherein the width of the spiral channel distributor decreases slowly butsteadily with increasing length, and the cylindrical area lying inbetween increases correspondingly, and the spiral channel distributorbeing bounded laterally by defined upper and lower edges, and the loweredge and the downwardly adjoining cylindrical surface being set backrelative to the upper edge.
 4. The storage head of claim 1, wherein thespiral channels have a semicircular cross-section at least at theirbeginning, and said semicircular cross-section becomes flatter withincreasing length, and the depth of the spirals decreases more.
 5. Thestorage head of claim 1, further comprising essentially radial boring(32, 32') leading into the shaft (12) for receiving plastic melt from atleast two extruders, said borings (32, 32') in said shaft (12) leadingto, at least two axial borings (22, 22') situated centrally within theshaft (12), and the individual spiral channels to be supplied withplastic melt are connected to the central borings (22, 22') respectivelythrough borings (30, 30') which run radially and which are disposed instar-shaped fashion.
 6. The storage head of claim 5, wherein the lengthof the respective axial borings (22, 22') situated centrally in theshaft (12), is at least as long as the diameter of the boring (22, 22')itself.
 7. The storage head of claim 5, wherein, for at least one axialboring (22, 22'), the radial boring (32, 32') is disposed on that sidethat is connected to the axial boring at an end thereof facing the lowerend of the ejector piston (14), and the star-shaped radial borings (30,30'), which lead to the spiral channels, are connected to the axialboring (22, 22') at the opposite end thereof which faces away from thelower end of the ejector piston (14), so that the plastic melt in saidcentral axial boring (22, 22') is conducted in a flow direction that isopposite to the ejection direction.
 8. The storage head of claim 1,wherein at least one of the setbolts (18), which connect on top to theejector piston (14), is hollow for conducting the plastic melt into adistributor element (20).
 9. The storage head of claim 8, wherein theconnection between the hollow setbolt (18) to the ejector piston (14),is by way of a spherically shaped connection defining a spherical head(48) and a spherical shell (50).
 10. The storage head of claim 1wherein, at least two adjoining distributor elements include spiralchannel distributors which are designed oppositely right-handed andleft-handed.
 11. The storage head of one of the preceding claims 1through 10, wherein, to form a multiple-layer plastic body, adistributor element with spiral channel distributors is provided foreach layer of plastic melt to be extruded, said channel distributorsbeing connected to at least two extruders, and wherein the ejectorpiston (14) includes concentric cylindrical tubular pieces (42, 44, 46),and the spiral-channel distributor (20) is disposed between theindividual cylindrical tubular pieces (42, 44, 46), the spirals of eachspiral-channel distributor being formed in the outside wall of eachtubular piece (42, 44, 46).